Fallen tree trunk is only found of the respective trees mentioned when walking across the biomes where they are found, not sure if this inconsistency is intended or not.

Steps to Reproduce:
1. Create a world
2. Find or locate a biome where each trees is found 
3. Look for the fallen trunks

Observed Results:
No acacia, dark oak, mangrove and cherry wood fallen trunk is found

Possibly Expected Results:
Like oak, birch, spruce and jungle trees, fallen tree trunks of these variants of trees generate too.